The present invention relates to methods for producing ruthenium perovskite having high purity and high crystallinity and having large specific surface area, and ruthenium perovskite catalyst obtained by these methods.
As a synthesis method of LaRuO3, which is a rare ruthenium perovskite type compound, composed of trivalent ruthenium ions, the following synthesis methods by solid phase reaction have been reported.
(1) 2La2O3+3RuO2+Ruxe2x86x924LaRuO3 (reaction conditions: at 1,350xc2x0 C. for 48 hours and at 1,000xc2x0 C. for 48 hours: J. Solid State Chem. 4,80,1972)
(2) 2La2O3+3RuO2+Ruxe2x86x924LaRuO3 (reaction conditions: under 2 GPa at 1,300xc2x0 C. for 30 minutes: Mat.Res.Bull. 29,1271,1994)
With these solid phase reaction processes, some quantity of ruthenium metal and RuO2 are often formed as by-products, and it is difficult to obtain high purity perovskite phase. Further, patents regarding production processes of said substance (including foreign countries"" patents) are mostly based on the above processes.
There are following problems in conventional synthesis methods of ruthenium perovskite.
(1) The production process has to be carried out at a high temperature and tends to be complicated, and the process cost is high.
(2) The obtained product tends to have a small specific surface area, due to a solid phase reaction and high synthesis temperature.
(3) These methods result in lack of purity and dispersion of lanthanum ruthenium perovskite when synthesized on a catalyst carrier or another carrier.
(4) These previous methods do not offer good mixing and contact of precursors/reactive ions, and therefore these precursors often react with the support materials, leading to the formation of undesired phases.
(5) The adhesive strength of a catalyst on a catalyst carrier or another carrier tends to be weak in most of the cases.
The present inventors have developed novel synthesis methods of ruthenium perovskite, which overcome the above problems, and have found that LaRuO3 having excellent physical and catalytic properties can be obtained by these methods.
Namely, the present invention provides a method for producing ruthenium perovskite represented by a chemical formula LaRuO3, wherein La is lanthanum to which twelve oxygen ions are coordinated, Ru is ruthenium to which six oxygen ions are coordinated, and O is oxygen, which comprises reacting an aqueous mixed metal ion solution containing metal ions of La and Ru, with a precipitate-forming liquid in a reaction container to co-precipitate hydroxides of La and Ru, and subjecting the precipitate to a heat treatment.
The present invention further provides the above method for producing ruthenium perovskite, wherein the precipitate is deposited, adhered or coated on a catalyst carrier or another carrier and then subjected to a heat treatment to form a coating of LaRuO3. The lanthanum and ruthenium metal in co-precipitated state offer close contacts with each other to form LaRuO3, rather than reacting with the support material. This also reduces the possibility of ruthenium getting oxidized to a higher valance state during the synthesis of LaRuO3.
The present invention further provides the above method for producing ruthenium perovskite, wherein the catalyst carrier or other carrier is coated with an alkaline earth oxide or a rare earth metal oxide.
The present invention further provides a method for producing ruthenium perovskite represented by a chemical formula LaRuO3, wherein La is lanthanum to which twelve oxygen ions are coordinated, Ru is ruthenium to which six oxygen ions are coordinated, and O is oxygen, which comprises directly heating an aqueous mixed metal ion solution containing metal ions of La and Ru in a reaction container, followed by evaporation to dryness.
The present invention further provides the above method for producing ruthenium perovskite, wherein part of La in the chemical formula LaRuO3 is replaced with another rare earth element.
The present invention further provides an innovative method to prepare a properly dispersed LaRuO3 on a catalyst or other support, using a xe2x80x9cdeposition-precipitationxe2x80x9d technique. This involves co-precipitation of La and Ru hydroxides on a support material, from a homogenous mixed metal ion solution, by slow and controlled decomposition of urea present in the same solution. The support containing co-precipitated precursors is subsequently heated to form a well dispersed LaRuO3 on powder, honeycomb or other catalyst carriers.
The present invention still further provides a ruthenium perovskite catalyst which comprises ruthenium perovskite obtained by the above production methods, which is used as a catalyst for oxidation reaction of hydrocarbon accompanied by no generation of carbon monoxide, as a catalyst for an oxidation reaction of carbon monoxide or as a catalyst for reduction reaction of NOx by carbon monoxide or hydrocarbon.
The production method of the present invention is roughly represented by the following reaction formulae. Here, in the following formulae, A is an anion component of a water-soluble material such as lanthanum or ruthenium, L is a precipitate-forming agent such as ammonia, sodium carbonate, potassium carbonate, urea or NaOH, and R is a residual component of the precipitate-forming agent.
Step of co-precipitation: La3++Ru3++A+L+H2Oxe2x86x92La.Ru.O.H.R (co-precipitated product, paste-like amorphous hydroxide)
Step of evaporation to dryness: La.Ru.O.H.Rxe2x86x92La.Ru.O.H2O (solid amorphous)
Step of dehydration and crystallization by a heat treatment: La.Ru.O.H2Oxe2x86x92LaRuO3 (perovskite crystal)
Step of particle size control and stabilization of crystal particles by a heat treatment at a high temperature: LaRuO3xe2x86x92LaRuO3 
The synthesis method of ruthenium perovskite of the present invention has the following characteristics as compared with conventional methods.
1) It can be carried out with relatively less process time and at lower temperature, thereby lowering the production cost.
2) A material having relatively large specific surface area and having good purity and crystallinity can be obtained.
3) A material can be directly synthesized with better purity, on a catalyst carrier or another carrier. Some of these methods offer reduced reactivity of precursors towards the catalyst carrier thereby producing supported LaRuO3 with better purity.
4) LaRuO3 material uniformly and minutely dispersed on a catalyst carrier or another carrier can be synthesized.
5) LaRuO3 more strongly adhered to a catalyst carrier or another carrier can be synthesized.
6) A material retaining a large specific surface area even after the carrier treatment, useful as a catalyst or for other applications, can be synthesized.
7) LaRuO3 having excellent catalytic properties for oxidation catalytic reaction of carbon monoxide or hydrocarbon or volatile organic compounds (VOCs) in the air or in exhaust gas; and for redox catalytic reactions of NOx by hydrocarbon or carbon monoxide, can be synthesized.
The ruthenium perovskite obtained by the production method of the present invention is useful, for example, for the following applications.
1) Removal of carbon monoxide and hydrocarbon discharged from gasoline engine or other sources, by oxidation catalytic action.
2) Removal of NOx discharged from gasoline engine or other sources, by hydrocarbon or carbon monoxide with a low temperature redox catalytic action.
3) Removal of volatile organic compounds(VOCs) from different sources, by catalytic oxidation reaction.
4) Clarification of the above air pollutants contained in the air by the above actions.
5) Preparation of supported and unsupported LaRuO3 for applications other than catalysis.
Now, the present invention will be described in detail with reference to the preferred embodiments.